The Mac machines have updated their SSH version and so the ssh-keygen
format has changed. Ask it for MD5, which is the one that is output as
hex.

This lets us see the details of what we're doing instead of just seeing the
output of unknown commands in the build output.

Coverity user model

The `git_buf` structure seems to be too complicated to correctly
grasp for Coverity. As such, add simpler models trying to guide
Coverity and remove false positives related to these functions.

The static analysis engine coverity allows for user models
overriding how it treats functions when analyzing code. Like
this, one can greatly reduce the rate of false positives and thus
make it easier to spot actual errors.

Add a user model that overrides function models for `git_buf_len`
and `git_vector_insert`, which together amount for a majority of
false positives.

It takes a bit for the propxy to get ready to accept connections, so
start it before the build so we can be reasonably sure that it's going
to be ready in time.